Such axial flow fans are generally provided with a plurality of blades, each of which is secured at its root to a hub that is driven by a rotating shaft and from which the blade extends radially outwardly. The blades can be spaced around the hub in a symmetrical or non-symmetrical fashion. Axial flow fans are known having blades of various designs. Thus, the blades can be provided with a tangential sweep either in the forward or rearward direction, with variations in pitch angle to suit particular applications. Furthermore, it is known to secure the blade tips to an outer circular band which encloses the blades and is generally centered on the axis of rotation of the fan.
When used in a vehicular application, the fan can be arranged either to blow air through a heat exchange system if the heat exchange system is on the high-pressure (downstream) side of the fan or draw air through the heat exchange system if the heat exchange system is on the low-pressure (upstream) side of the fan. Such fans can be made from moulded plastics or from sheet metal or a combination of the two.
The performance of the fan is of particular concern when used to cool air in an enclosed engine compartment. More particularly, it is required to reduce the noise generated by such fans without a reduction in their performance and efficiency. Another requirement is that the fan should be strong enough to resist the stresses applied to it at high flow rates, and in adverse operating environments.
Reference is made to the following documents which describe fans designed particularly for vehicular cooling applications.
U.S. Pat. Nos. 4358245, 4569631 and 4569632 disclose a fan of the general type with which the present invention is concerned and which has blades which are skewed forwardly or rearwardly or have a combination of forward and rearward skews to improve efficiency and reduce noise. GB-A-2178798 describes a fan having blades with a relatively more forwardly curved outer portion, to reduce noise.
A first object of the present invention is to provide a fan having greater mechanical strength without loss of efficiency and flow performance characteristics as compared with the fans described in these prior art documents.
A second object of the invention is to provide a fan exhibiting lower noise.